This application relates to Quantum Well Infrared Focal Plane Arrays, and more particularly, to Type-II Superlattice Focal Plane Arrays.
Infrared focal plane arrays (FPA) have found many applications that can be divided into two major categories: military and commercial. Military applications include night vision, target acquisition and tracking, reconnaissance, fire control, etc. Commercial applications include industrial, environmental, civil, and medical. Both infrared thermal detectors and photon detectors can be used to fabricate FPAs.
As is known in the art, QWIP FPAs are composed of arrays of detector structures, wherein each detector structure produces a signal that is transmitted through a conductor bump to an external Read Out Integrated Circuit (ROIC) unit cell. The outputs of the plurality of ROIC unit cells associated with each detector in the array produce an integrated representation of the signal from the detector. To produce this output signal, a fixed bias is applied to the detector and the detector photocurrent resulting from the bias and the incident radiation is integrated. This integration function is performed by an integration charge well (integration capacitor) that is disposed within each individual ROIC unit cell. The combined integrated outputs of the plurality of ROIC unit cells in the array produce an image corresponding to the received infrared radiation.
Most quantum well and superlattice photon detectors are grown by state-of-the-art MOCVD and MBE techniques which allows a high material uniformity across a larger wafer. Higher material uniformity translates to FPAs with a higher detector array uniformity, which is very important for lower NEΔT (noise equivalent temperature difference). Due to its internal detection mechanism: absorption of photons by electrons, a quantum well photon detector has very fast response time (up to 30 GHz) compared to thermal and other infrared detectors. Fast response time FPAs are highly favored by military applications such as target tracking. With quantum well structure engineering, it is not difficult to tune the detection wavelength of quantum well photon detectors and, by stacking quantum well structures, it is also not difficult to achieve multi-band detection.
Compared with thermal detectors, such as microbolometers, pyroelectric detector, quantum well photon detectors generally have higher detectivity, which translates to a FPA with much lower NEΔT.
Type-II superlattice detectors (i.e. quantum well photon detectors with type-II band alignment) have shown high room temperature detectivity and such quantum well detectors can be used to build future uncooled FPAs with fast response. So far, most uncooled FPAs are based on slow thermal detectors, such as microbolometers.
With the advances of III–V semiconductor material growth such as multiquantum well (MQW) structures on a Silicon substrate, a quantum well photon detector can be grown on a Si wafer, thereby permitting a monolithic infrared FPA integrated with a Si based ROIC. With a monolithic scheme, low cost FPA is expected due to the elimination of the complicated hybridization process, which is currently necessary in the hybrid infrared FPA fabrication.
The compositions of lattice-matched semiconductor materials of the quantum well layers can be adjusted to cover a wide range of wavelengths for infrared detection and sensing. Quantum-well structures can achieve, among other advantages, high uniformity, a low noise-equivalent temperature difference, large format arrays, high radiation hardness, and low cost.
Each MQW structure has multiple quantum wells which are artificially fabricated by alternatively placing thin layers of two different, high-bandgap semiconductor materials adjacent to one another to form a stack, as known in the art. The bandgap discontinuity of the two materials creates quantized subbands in the potential wells associated with conduction bands or valence bands.
The band alignment of any heterojunction can be categorized as type-I, type-II staggered or type-II misaligned. In type-I heterojunctions, one material has lower energy for electrons and the holes and therefore both carriers are confined in that layer. In type-II heterojunctions, however, the electrons are confined in one material and the holes in the other. In the extreme case, which is called type-II misaligned, the energy of the conduction band of one material is less than the valence band of the other one.
Type-II superlattice detectors are based on interband optical transitions and hence they can operate at much higher temperatures. Moreover, theoretical calculations and experimental results show that InAs/Gal1-xInxSb type-II superlattices have a similar absorption coefficient to HgCdTe, and therefore type-II superlattice detectors with high quantum efficiencies are possible.
The special band alignment of the type-II heterojunctions provides three important features that may be used in many devices to improve the overall performance of the device.
The first feature is that a superlattice with the type-II band structure can have a lower effective bandgap than the bandgap of each layer. This is an important issue for the applications in the mid and long infrared wavelength range, since one can generate an artificial material (the superlattice) with a constant lattice parameter but different bandgaps. Very successful detectors and lasers have been implemented in the 2–15 μm wavelength range and InAs/GaInSb superlattices lattice-matched to GaSb substrates.
The second feature is the spatial separation of the electrons and holes in a type-II heterojunction. This phenomenon is a unique feature of this band alignment and is due to the separation of the electron and hole potential wells. As a result of such spatial separation, a huge internal electrical field exists in the junction without any doping or hydrostaticpressure. High performance optical modulators have been implemented based on this feature.
The third feature is the zener-type tunneling in a type-II misaligned heterojunction. Electrons can easily tunnel from the conduction band of one layer to the valence band of the other layer, since the energy of the conduction band of the former layer is less than the energy of the valence band of the later layer. Unlike a zener tunneling junction which requires heavily doped layers, no doping is necessary for such a junction. Therefore, even a semimetal layer can be implemented with very high electron and hole mobilities since the impurity and ion scattering are very low. This feature of type-II heterojunctions has been successfully used for resonant tunneling diodes (RTDs) and recently for the implementation of type-II quantum cascade lasers.